[gn] Set correct defaults for some gn args

BUILD.gn

 # Copyright 2014 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 import("//build/config/android/config.gni")
 import("//build/config/arm.gni")
 import("//build/config/dcheck_always_on.gni")
 import("//build/config/mips.gni")
 import("//build/config/sanitizers/sanitizers.gni")
 
 if (is_android) {
   import("//build/config/android/rules.gni")
 }
 
 import("gni/v8.gni")
 import("gni/isolate.gni")
 import("//build_overrides/v8.gni")
 
 import("snapshot_toolchain.gni")
 
 declare_args() {
   # Sets -DVERIFY_HEAP.
   v8_enable_verify_heap = false
 
   # Enable compiler warnings when using V8_DEPRECATED apis.
   v8_deprecation_warnings = false
 
   # Enable compiler warnings when using V8_DEPRECATE_SOON apis.
-  v8_imminent_deprecation_warnings = false
+  v8_imminent_deprecation_warnings = ""
 
   # Embeds the given script into the snapshot.
   v8_embed_script = ""
 
   # Support for backtrace_symbols on linux.
   v8_enable_backtrace = false
 
   # Sets -dENABLE_DISASSEMBLER.
   v8_enable_disassembler = false
 
   # Sets -dENABLE_GDB_JIT_INTERFACE.
-  v8_enable_gdbjit = false
+  v8_enable_gdbjit = ""
 
   # Sets -dENABLE_HANDLE_ZAPPING.
   v8_enable_handle_zapping = true
 
   # Enable ECMAScript Internationalization API. Enabling this feature will
   # add a dependency on the ICU library.
   v8_enable_i18n_support = true
 
   # Enable slow dchecks.
   v8_enable_slow_dchecks = false
@@ skipping 10 matching lines @@
   # tools/gen-postmortem-metadata.py for details.
   v8_postmortem_support = false
 
   # Similar to vfp but on MIPS.
   v8_can_use_fpu_instructions = true
 
   # Similar to the ARM hard float ABI but on MIPS.
   v8_use_mips_abi_hardfloat = true
 }
 
+# Set project-specific defaults for some args if not provided in args.gn. The
+# defaults can be set in the respective build_overrides files.
+if (v8_imminent_deprecation_warnings == "") {
+  if (defined(v8_imminent_deprecation_warnings_default)) {

[Michael Achenbach, 2016/07/15 11:33:17]

Without guarding with the defined check, we'd need to provide upstream changes
to chromium and pdfium first, which I find more brittle.

+    v8_imminent_deprecation_warnings = v8_imminent_deprecation_warnings_default
+  } else {
+    v8_imminent_deprecation_warnings = false
+  }
+}
+if (v8_enable_gdbjit == "") {
+  if (defined(v8_enable_gdbjit_default)) {
+    v8_enable_gdbjit = v8_enable_gdbjit_default
+  } else {
+    v8_enable_gdbjit = false
+  }
+}
+
 v8_random_seed = "314159265"
 v8_toolset_for_shell = "host"
 
 ###############################################################################
 # Configurations
 #
 config("internal_config") {
   visibility = [ ":*" ]  # Only targets in this file can depend on this.
 
   include_dirs = [ "." ]
@@ skipping 2317 matching lines @@
 
   deps = [
     ":fuzzer_support",
   ]
 
   configs = [ ":internal_config" ]
 }
 
 v8_fuzzer("wasm_asmjs_fuzzer") {
 }